FIG. 6 shows a typical conventional bicycle rear derailleur.
This bicycle rear derailleur 1 is arranged so that by using a control mechanism such as a pantograph link mechanism 2, a chain guide 5 having a guide pulley 3 and a tension pulley 4 can be moved axially of a shaft of a multiple sprocket F.
The pantograph link mechanism 2 has a link base 7 to attach to an end plate 6, for example, of a bicycle frame, a forwardly extending pair of inner and outer link members 8, 9, each pivoted at a base end thereof to the link base 7 so as to move laterally of the bicycle, and a movable member 10 pivoted to respective free ends of the inner and outer link members 8, 9. Each of the constituent members 7, 8, 9, 10 of this pantograph link mechanism 2 are arranged and pivoted in a shape of deformable parallelogram, so that when the pantograph link mechanism 2 is deformed, the movable member 10 is translated laterally of the bicycle.
Since the chain guide 5 is mounted to the movable member 10, it is also translated laterally of the bicycle with the movable member 10 as the pantograph link mechanism 2 deforms.
While the bicycle is running, a chain C runs on a rear side of the tension pulley 4, a front side of the guide pulley 3, and then, a rear side of a sprocket of the multiple sprocket F. Therefore, when the guide pulley 3, which supports a portion of the chain immediately before engaging the multiple sprocket F, is moved axially of the multiple sprocket F as described earlier, the chain C is shifted onto another sprocket which positionally corresponds to the moved guide pulley 3.
As described hereinabove, in the bicycle derailleur, the guide pulley 3 is directly responsible for shifting the chain C, and in order to improve shifting response of the chain C, it is necessary for this guide pulley 3 to be positioned at an appropriate distance, not too close to or too far away, from the multiple sprocket F.
Each of the sprockets constituting the multiple sprocket F is arranged so that a diametrically smallest sprocket is placed at a laterally outermost position, and a diametrically larger one is placed more inward laterally of the bicycle. In an attempt to position the guide pulley 3 at an appropriate distance from each of these diametrically different sprockets, a number of proposals have been made conventionally.
One of these is disclosed in the Japanese Patent Publication Sho 42-23486, wherein pivot pins which link the constituent members of the pantograph link mechanism are slanted inward laterally of the bicycle. This is commonly known as the slant pantograph link mechanism. With this arrangement, when the pantograph link mechanism is deformed to move the chain guide inward laterally of the bicycle, the chain guide moves with inclination, or downwardly as well as inwardly. Since this inclination corresponds to an inclination of a line provided by connecting the outer edges of respective sprockets of the multiple sprocket, it is possible to reduce variation in the distance between the guide pulley and respective corresponding sprockets.
There is a problem however: Since the moving direction of the movable member of pantograph link mechanism is fixed, there is no adaptability to different sprocket configurations, for example, when the multiple sprocket is replaced from what is known as a close-ratio type to a wide-ratio type, or in other words, from a sprocket configuration wherein a gear teeth ratio between sprockets is small to another configuration wherein the same is large.
In an attempt to solve this problem, the Japanese Patent Publication Sho 62-10874 discloses an invention, wherein the slant angle of pantograph link mechanism is set for an interchangeable multiple sprocket of the widest possible ratio whereas the link mechanism is pivotally mounted to the bicycle frame, being urged in a direction to tension the chain. This mechanism, in which the link mechanism is pivotally mounted to the bicycle frame and is urged, is referred to as the double tension mechanism since the chain guide pivoted on the link mechanism is also urged, as a matter of course, in a direction to tension the chain.
Hence, the bicycle rear derailleur disclosed in the Japanese Patent Publication Sho 62-10874 is a combination of a slant pantograph link mechanism and a double tension mechanism, wherein the slant angle for the pantograph link mechanism is set to correspond to a multiple sprocket of the widest possible ratio.
The above Patent Publication describes that in a rear derailleur arranged as above, when the chain is shifted from a diametrically smaller sprocket to a diametrically larger sprocket, the pantograph link mechanism deforms to move the guide pulley away from the multiple sprocket; however, a spring which urges the link mechanism is compressed to pivotally move the link mechanism counterclockwise to adjust the position of the guide pulley closer to the sprocket, thereby maintaining a constant distance between the guide pulley and each sprocket regardless of the configuration of the multiple sprocket.
However, there are still the following problems with this particular bicycle derailleur disclosed in the Japanese Patent Publication Sho 62-10874:
First, when the slant angle of the pantograph link mechanism is set to have an inclination which corresponds to the case wherein the multiple sprocket has a sprocket configuration of the widest possible gear ratio, a considerably large stroke must be allowed for the movable member. Hence, the link mechanism needs to be increased in size. In addition, the link mechanism must be deformed substantially, resulting in a deteriorated operation efficiency to deform the link mechanism.
Second, use of the double tension mechanism inherently poses a problem that as the chain is shifted onto a diametrically larger sprocket, the guide pulley tends to come too close to that diametrically larger sprocket. This tendency is more intense as the multiple sprocket is of a wider ratio. Practically therefore, on a multiple sprocket of a wide ratio, chain shifting efficiency is not so good when the chain is shifted to a diametrically larger sprocket.
This inherent problem in the conventional double tension mechanism will now be elaborated referring to FIG. 6.
As shown in FIG. 6, the link mechanism 2 extends forwardly from the link base 7 which is the base end side; free ends thereof supports the movable member 10 which supports the chain guide 5. This link mechanism 2 is urged by a spring 11 for tensioning the chain C in a clockwise direction, or in other words in a direction for the guide pulley 3 to move away from the multiple sprocket F.
In the link mechanism arranged as hereinabove, as the chain C is shifted onto a diametrically larger sprocket, chain becomes less tense and therefore, the tension pulley 4 is pulled forwardly to move the chain guide 5 in a counterclockwise direction. In this case, the chain guide 5 being urged by the spring 12 compresses this spring as it moves.
However, when the chain guide 5 is moved counterclockwise compressing the spring 12, one effect is that the link mechanism 2 is moved counterclockwise to compress a spring 11 which urges the link mechanism. In other words, as the chain C is moved to a diametrically larger sprocket, the guide pulley comes closer to the sprocket. This tendency is more significant in the multiple sprocket having a wider gear ratio. As a result, it becomes likely that the chain guide 5 or the movable member 10 interferes with the diametrically larger sprocket to hinder speed change operation.
Therefore, in order to solve this problem, the slant angle has to be large, or the distance between the guide pulley and a diametrically smaller sprocket has to be wide enough to allow for clearance between the guide pulley and a diametrically larger sprocket.